[codex] Add OTel request observability

[sorenbs]

Summary

Adds first-class request observability for Streams, including the otel-traces profile, OTLP normalization/ingest support, explicit observability pairing metadata, and the /v1/observe/request correlation API used by Studio.

The implementation also extends the evlog profile with correlation/search metadata, adds request observability docs and spec updates, and hardens diagnostics around coverage, limits, policy enforcement, and event-derived OTel errors when raw events are excluded.

Why

Studio needs a supported backend contract for correlating application events with traces without choosing counterpart streams heuristically or relying on ad hoc search behavior. The new profile-owned pairing descriptor and observe API make that contract explicit, searchable, bounded, and documented.

Impact

• Adds otel-traces profile support and OTLP request/span normalization.
• Adds request observability pairing discovery for compatible event and trace streams.
• Adds /v1/observe/request for requestId, traceId, and spanId lookups with bounded result limits and coverage diagnostics.
• Adds docs for profile behavior, request observability, architecture, spec, README, and stream profile surfaces.
• Adds focused unit/integration coverage, including scale-oriented observe request coverage.
• Fixes local verification ergonomics around shutdown and repo-wide test/lint behavior.

Validation

Validated locally on the latest branch state:

• bun run check:result-policy
• bun run typecheck
• bun test

Final full test run passed with 363 passing tests, 8 skipped, and 0 failures.